by Tas Walker Ph.D.
A large sheet-like igneous formation in central Australia was once thought to have been emplaced into its host rocks after they lithified, or to have been deposited on the surface as a welded pyroclastic flow. More detailed field work has shown it to have been emplaced into its host sediments while they were unconsolidated and saturated with water. This new synsedimentary interpretation is based on tell-tale characteristics of magma/sediment interaction at the sill margins, including the presence of peperite. Other large igneous bodies have similarly been found to show evidence of significant magma/sediment interaction in a watery environment. These new discoveries provide clear evidence of rapid, large-scale watery catastrophe consistent with a geologic model based on Noah’s Flood from the Bible.
In 1993, Jocelyn McPhie of the University of Tasmania published a study on the extensive Tennant Creek porphyry formation surrounding Tennant Creek, a small mining town in central Australia.1 She identified peperite texture at the margins of the porphyry sill where it contacts thick sequences of enclosing sedimentary sandstone and siltstone. Her paper provoked responses in the geological literature,2,3 not challenging her interpretation of peperite, but discussing the problem of how the rock formation could possibly have been emplaced within the sediment.
Peperite is not named after the hot seasoning used at the dining table, but after a town in France called Peper, near which characteristic deposits of peperite are described.4 Peperite refers to the rock that forms when hot lava erupts into wet and unconsolidated sediment. This interaction produces a deposit that is neither sedimentary rock nor volcanic rock, but a true mix of the two. As a mixture of sediment and magma, peperite is recognized by its texture.
White, McPhie and Skilling5 define peperite as:
‘a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet sediments. The term also refers to similar mixtures generated by the same processes operating at the contacts of lavas and hot pyroclastic flow deposits with such sediments’ [emphasis in original].
As shown in figure 1, peperite forms in association with dikes and other intrusions and at the base of lavas flowing over the surface of wet sediments. Thus, peperite is a genetic term connected to a particular emplacement process….
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